Light emitting devices are used in many applications including optical communication systems. Optical communication systems have been in existence for some time and continue to increase in use due to the large amount of bandwidth available for transporting signals. Optical communication systems provide high bandwidth and superior speed and are suitable for efficiently communicating large amounts of voice and data over long distances. Optical communication systems that operate at relatively long wavelengths on the order of 1.3 micrometers (μm) to 1.55 μm are generally preferred because optical fibers generally have their lowest attenuation in this wavelength range. These long wavelength optical communication systems include a light source capable of emitting light at a relatively long wavelength. Such a light source can be, for example, a vertical-cavity surface-emitting laser (VCSEL), an edge-emitting laser, or other types of light sources.
These light sources include an active region into which carriers, i.e., electrons and holes, are injected. The holes and electrons recombine in the active region and emit coherent light at a particular wavelength. One manner of forming an active region in a light emitting device is to form a quantum well layer and sandwich the quantum well layer between a pair of adjacent barrier layers. The quantum well layer and the adjacent barrier layers form what is referred to as a quantum well. The quantum well layer typically comprises a low bandgap semiconductor material, while the barrier layers typically have a bandgap higher than the bandgap of the quantum well layers. In this manner, when the laser diode is subject to forward bias, electrons and holes are injected into and trapped in the quantum well layer and recombine to emit coherent light at a particular wavelength. Generally, more than one quantum well is formed in a light emitting device. The optimum number of quantum wells is dependent upon the material system from which the quantum wells are grown and on the required optical gain
A light emitting device has a threshold current (Ith), which is the current at which lasing action begins. The relationship between temperature and threshold current of a light emitting device is exponential, and can be characterized by the formula Ith ∝ exp T/T0, where T0 is the characteristic temperature of the light emitting device.
A quantum well layer for a 1.5 μm wavelength light emitting device can be formed using indium gallium arsenide phosphide (InGaAsP), which can be formed over an indium phosphide (InP) substrate. Unfortunately, for a conventional light emitting device having an InGaAsP quantum well layer, the value of T0 is small, resulting in a rapid increase in the value of Ith when temperature rises. This occurs mainly due to Auger recombination and carrier leakage, as known to those skilled in the art. Therefore, InGaAsP quantum well layers are not particularly well suited for 1.5 μm wavelength output light emitting devices in which a low threshold current and high characteristic temperature are desired.
A quantum well layer for a 1.3 μm wavelength light emitting device can be formed using InAsP, which can be formed over an indium phosphide (InP) substrate, and which has a higher characteristic temperature, T0, than InGaAsP. Depending on the arsenic fraction of a quantum well layer formed using InAsP, the operating wavelength of a light emitting device can be extended to approximately 1.3 μm. However, it would be desirable to extend the wavelength in which an InAsP quantum well layer generates photons to approximately 1.5 μm. Obtaining such an output wavelength from an InAsP quantum well layer suggests that the arsenic fraction in the InAsP layer approach 60%. Unfortunately, when using conventional processing techniques, such an arsenic fraction results in a significant lattice mismatch when the InAsP is grown over InP. The lattice mismatch can approach 2%. Thus, the InAsP quantum well layers are highly strained. These highly strained quantum well layers may relax during, or after their formation, thereby resulting in the formation of dislocations in the InAsP layer. Dislocations are stress fractures in the epitaxial film and can degrade the optical performance of the material by destroying the material's luminescence efficiency, sometimes referred to as photoluminescence intensity, thereby making the material unacceptable for use in a light emitting device.
Forming an InAsP layer is possible using a technique known as organometallic vapor phase epitaxy (OMVPE), sometimes referred to as metal organic chemical vapor deposition (MOCVD). OMVPE uses liquid or solid chemical precursors, through which a carrier gas is passed, to generate a chemical vapor that is passed over a heated semiconductor substrate located in a reactor. Conditions in the reactor are controlled so that the combination of vapors forms an epitaxial film as the vapors pass over the substrate. OMVPE is an economical and well established technology for growing epitaxial films.
Unfortunately, as mentioned above, growing high optical quality InAsP is difficult because, when using conventional growth parameters, the arsenic fraction required for light emission at 1.5 μm results in dislocations in the epitaxial material sufficient to render the material unusable for a light emitting device.
Therefore, it would be desirable to economically mass produce a long-wavelength light emitting device having an InAsP quantum well layer using OMVPE.